1. Field of the Invention
The present invention relates to back-lit image sensors and, more specifically, to a method for establishing a contact, from the rear surface, with a metal track formed on the front surface of a back-lit image capture device.
2. Discussion of the Related Art
In back-lit image sensors, photodiodes and transfer transistors are formed on the front surface of a substrate and an interconnect stack comprising many vias and metal track levels is formed on the front surface of the substrate to connected the photodetection elements together in adapted fashion. A support is attached to the interconnect stack and the substrate is thinned down to enable a lighting of the photodiodes from the rear surface, through the thinned-down substrate.
To connect a metal track formed in the interconnect stack to an element external to the circuit, it is generally provided to contact the metal track from the rear surface of the circuit, through the thinned-down substrate and next to the photodetection areas. Indeed, it would be difficult to contact the front surface of the device due to the thickness of the support on which the interconnect stack is attached.
Many methods have been suggested to contact a metal track present in the interconnect stack from the rear surface of the circuit. However, these methods have several disadvantages. First, they generally provide a succession of several etch steps, which necessitates the forming of several masks. The implementation of such methods is thus relatively long. Further, some at least of the known methods imply the stopping of at least one of the etchings directly on the metal layer of the interconnect stack which is closest to the substrate, which poses problems of corrosion of the material forming this track.